A conventional Soxhlet extraction apparatus, shown in FIG. 1, has been used in the prior art for the continuous extraction of a solid by a hot solvent. In operation, the Soxhlet extraction apparatus holds a solid substance 11 in thimble 10 which is placed within the inner tube of the apparatus. At one end, the Soxhlet apparatus is fitted to flask 12 which contains heated solvent and at its other end the apparatus is connected to reflux condenser 13. As the solvent in flask 12 is boiled, the vapor passes up through tube 14 and is condensed by condenser 13 so that the condensed solvent falls into thimble 10. When the solvent reaches the top of tube 15, it is siphoned over into flask 12 and thereby removes that portion of the substance which it has extracted in thimble 10. The process is repeated automatically until complete extraction is effected. The extracted compound may be isolated from its solution in flask 12 by a variety of known methods.
There are several advantages to the conventional Soxhlet extraction apparatus and method. First, a low total solvent volume is needed (relative to the sometimes high number of Soxhlet cycles required). Solvent conservation also occurs through the continual reuse of the solvent or solvent system, since essentially only the necessary extraction zone fill volume plus a requisite excess, to keep from running out of solvent in the distillation flask, is recycled. Second, rather precise extraction volumes and solvent-to-substrate ratios for a given extraction operation can be obtained.
However, there are many disadvantages which need to be overcome or eliminated to allow achievement of improved results and/or allow high speed extraction. A first disadvantage is the requirement that one must use an adequate excess volume of the solvent or solvent system of choice in the distillation flask as compared to merely the volume necessary to fill the extraction zone to the siphon level. The additional solvent is required in order to prevent the burning of the substrate extractables of interest, because of running out of solvent in the distillation flask while trying to fill the extraction zone. Secondly, in order to change solvent or solvent systems, the Soxhlet apparatus setup must be dismantled when the requisite number of extraction cycles are completed and the distillation flasks must be switched. This also involves replacing the flask containing the solvent or solvent system no longer needed with a flask containing the next solvent or solvent system of choice.
A further disadvantage is the need to wait to catch the extraction zone just as it is finished dumping so that the distillation flask containing the solvent or solvent system no longer needed, before redistillation of its contents begins, can be removed. A still further disadvantage is the need to turn off the distillation flask heat source and to couple this with the finish of the extraction zone solvent dumping in such a way as to prevent the initiation of solvent drip-back fires when the distillation flask is removed.
It is essential that the substrate sample be removed from the extraction zone in such a manner that the solvent or solvent system not siphoned off in the dumping step (and it is typical that this volume in many instances may reach as much as 25% of the extraction zone volume) can be transferred analytically to the distillation flask since it contains a portion of the extractables that is part of the overall extractables picture. The substrate must also be squeezed or drained in such a manner as to transfer its surface solvent or solvent system/extractables mixture in an analytical manner to the distillation flask since it contains extractables that are also part of the overall extractables picture.
Another disadvantage is the need to readjust the heat source to the proper thermal input for the new solvent or solvent system of choice once the flask containing the new solvent or solvent system of choice has been installed on the Soxhlet extractor setup. Since the time required to complete one Soxhlet extraction cycle (cycle time) is usually the result of many interacting factors such as heat source variations, room temperature variations, condensation cooling source variations, distillation properties of the solvent or solvent system of choice, sample substrate packing configuration variations, etc., it is necessary, for each Soxhlet setup, to time the cycle after distillation/extraction equilibrium is reached (which involves the heating up of the apparatus) for one "representative" cycle and then distill/extract for the required amount of time to insure completion of the required number of extraction cycles which based on experience with the particular system involved is enough to remove all of the extractables of interest.
In order to insure that the extractables of interest are removed completely, a relatively high number of Soxhlet cycles must be run in most cases, which typically may range from 10 to 20 depending upon the conditions surrounding the extractions. The process not only often removes solubles from the substrate but also insolubles in various forms such as crystals, particles, fibers, etc. If these particulate materials are not accounted for, they cloud in various ways the soluble extractables picture arrived at in gravimetric work subsequent to the Soxhlet extractions. If they are accounted for they require that separation techniques and additional gravimetric procedures be incorporated in the analytical scheme.
It would not be unusual that with certain solvent system/extraction conditions/substrate extractable combinations, that some extractables components would be extracted, siphoned off and in some instances have distillation properties that would allow them to co-distill with the solvent or solvent system of choice thereby giving potentially misleading results due to limiting recontamination, etc., of the substrate to be extracted. Since the Soxhlet extraction apparatus is open, the extractions of interest must be carried out with solvents or solvent systems which distill under relatively standard atmospheric conditions.
The solvent/substrate interface wetting (aside from chemical considerations which are of primary significance) which occurs in the extraction zone is largely the result of two additional important forces and they are atmospheric diffusion processes and channelling effects due to substrate sample packing configurations which may vary from sample to sample. The fact that the solvent movement is related significantly to gravity and diffusion effects can detrimentally enhance the effects of solvent channelling.
It would not be unusual for the filling of the extraction zone, to the siphon drain off level in a conventional Soxhlet extraction device, to take an average time which might range from approximately 20 to 60 minutes. In these cases, it is also not unusual to observe differentials in the gradient of extracted materials in the solution and/or mixture from the top to the bottom of the extraction zone. In addition to the accumulative contributions to that gradient, from the washing of extractables from the top of the sample being extracted to the bottom of the extraction zone, the gradient formation is further enhanced in that significant, real, and relatively long extraction immersion exposure time differentials exist for the sample portion at the top of the extraction zone versus that for the sample portion at the bottom of the extraction zone before siphoning off occurs. These detriments necessitate the running of a sufficiently high number of extraction cycles to insure that most of the extractables of concern are removed adequately from the entire sample in a Soxhlet extraction exposure.
The temperature at which the extraction actually occur in the extraction zone is significantly lower than the distillation temperature of the solvent or solvent system used since much heat is removed from the distilled vapors in the condensation zone. Therefore, the extraction operation is limited to the resultant condensed temperature of the solvent system used and is confined to the variations which occur in that temperature. This means that for the solvent system of choice, extractions at a higher or lower range of temperature values than the resultant limits imposed by the system of choice cannot be performed.
And finally, inherent to the use of a Soxhlet extraction system is the fact that it can only be used for extraction work. Thus, it can only be used to extract with clean solvents or solvent systems. It follows therefore that it cannot be used to expose the substrate of interest to prepared or resultant process solutions or mixtures for the purposes of altering the substrate such as dissolving all or a part of it, cleaning its surface, adding surface materials, or the thorough penetration of it to remove, add, and/or alter materials of interest.
While the above-described mode of Soxhlet extractions is of utility in analytical work, the disadvantages noted herein with the use of conventional Soxhlet extraction devices usually require long extraction times, and the results are restricted to a relatively narrow band of extraction temperatures which are related to the properties of the solvent or solvent system of choice.